Pocket Guide: Why is My Pool Green?

If you’re a pool owner, you’ve likely thought the unfortunate but common question: “Why is my pool green?” Regardless of the cause, it’s best to not swim in pool water that has gone green or while it’s undergoing a process to treat the cause of the water’s greenness. To sanitize and balance pool water most likely requires adding strong concentrations of pool chemicals.

Let’s look at why swimming pool water turns green and solutions to the problem.

Why Is My Pool Green?

Though the most likely cause is algae, other causes might include rain, metals suspended in the water, or even corrosion. To determine the exact cause and keep from future wondering why the pool water is green, let’s look at the indications, and solutions, for each.


Green discoloration caused by algae spreads rapidly, especially in the heat of summer. This is due to an imbalance in the swimming pool’s chemistry. Even a mild case can bloom overnight, and letting chlorine levels drop for a day or two will turn a pool green quickly. Though in some cases, it may be possible to resolve the problem with some pool shock, if it’s beyond the point of prevention, this will take a bit more work. Now, let’s look at the steps to take to resolve the problem, after which we’ll look at how to prevent it from happening in the future.

To remove green discoloration from pool water, take the following steps:

  • Test the pool’s chemistry to get an idea of how the pool’s chemistry should be adjusted, though this may be done after vacuuming and brushing the pool.
  • Vacuum the pool to remove algae and other debris while also ensuring that the vacuum filter is set to remove this waste.
  • Brush the pool with special algae brushes designed for removing algae, which have stiffer bristles; while brushing after vacuuming may seem counterintuitive, the algae must remain in the pool for it to be killed off.
  • Shock the pool to kill the algae with a concentrated chlorine cleaner, which should be higher than 70 percent; non-chlorine pool shocks won’t work for algae.
  • Run the pool filter immediately afterward to ensure the chemicals circulate, usually about eight hours.
  • Test the pool’s chemistry again to ensure it’s been properly rebalanced, especially keeping an eye on alkalinity and pH.
  • To keep the algae from returning, it’s important to clean the pool filter, either by backwashing for sand or DE filters or with a chemical rinse for cartridge filters.

There are several ways to keep the algae from coming back. To prevent algae in a swimming pool: 

  • Shock the pool weekly.
  • Clean and sanitize pool equipment, including anything that goes into the pool, before it goes back into the water.
  • Wash, don’t rinse, all swimsuits that have been in and will go back into the pool.
  • Check for cracks, breaks, or other places where algae can grow; this may require some spot maintenance or indicate a need to resurface the pool.

Another method to keep a pool clear of algae involves algaecide. While it’s better to use chlorine shock to resolve this problem, algaecide helps prevent it. Once a pool has been cleaned and shocked, let chlorine levels fall to under 5 ppm (parts per million) before dosing the pool water.


This is also a common cause of why pools turn green and may even introduce algae spores into a swimming pool. In cases where it isn’t directly related to algae, however, the green color results from the loss of chlorine when pool water is diluted. Often runoff will additionally introduce dirt, nitrates, phosphates, and other organic material that counteracts chlorine.

This situation is resolved by:

  • Emptying the skimmer and pump baskets.
  • Removing floating debris from the pool surface with a leaf rake.
  • Pushing debris into the shallow end of the swimming pool that the net can’t easily get.
  • Testing and balancing pool chemicals.
  • Adding pool shock.

When removing bigger debris from the pool, it also agitates the water. If there’s dirt in the pool or smaller detritus, this can be vacuumed out of the pool via backwashing. Sometimes, however, it takes just removing by hand. To help prevent stuff from getting into the pump baskets or skimmer, it’s best to shut the pool pump down before any sort of major storm where high winds are expected.


Metals can find their way into swimming pools from several sources, but copper is usually the culprit. This may be due to copper in the pool’s water source, eroded copper piping, or copper algaecide. Used for many decades, copper algaecides kill most algae effectively. However, large doses of copper algaecide will turn water green as its fine particulates become suspended in water. While most of these algaecides contain chelating agents, these lose effectiveness after a while, so it requires a metal inhibitor to remove the copper and the green from the water.


When pool water turns green for this reason, it’s a serious situation triggered primarily due to an imbalance in the water, such as low alkalinity, calcium, or pH. When water becomes too acidic, it dissolves metals, turning them into ions. Acidic pool water will also cause swimmers’ eyes to burn and skin to itch.

It’s also the reason pool heaters leak and fails. Older pools sometimes utilize copper plumbing, which, when exposed to acidic conditions, will degrade and cause a pool’s water to turn green. Handrails, light fixtures, and anything made from stainless steel will start turning black. This is especially true with copper, which turns green when it corrodes. Shocking a pool in such acidic conditions is also the most common cause of pool staining.

To fix this problem, the pool either needs to be drained or treated. Treating requires adding some kind of chemical to prevent metal ions from forming into a precipitate, which results when an insoluble solid develops out of a liquid solution. Draining the pool will require a sump pump, though this should be drained into the sewer system rather than used on plant life, as chemicals and salts in pool water will stunt or kill plants.

Contact the Experts at Halogen Supply

Halogen Supply has offered residential, commercial, and institutional pool and spa services and products for over 80 years. We’ve built a reputation on excellent service, knowledge and product selection. So, if you are still wondering, “Why is my pool green?” we invite you to contact Halogen Supply today!